1. Field of the Invention
The present invention is in the area of lubrication of rolling/sliding surfaces, such as bearings and gears that may be required to operate over a large range of kinematic, stress and environmental conditions. Lubrication is conventionally supplied by liquid, solid, and sometimes gaseous materials, which provide a low shear strength film between the moving surfaces. The most reliable lubricating films are those generated by hydrodynamic (HD) or elastohydrodynamic (EHD) mechanisms, which separate the surfaces by a pressure buildup due to the pumping action of a fluid drawn into a convergent geometric region. The motion of a fluid drawn into a convergent zone by the moving surfaces generates sufficient pressure to cause a separation of the surfaces even under extremely high contact pressures. The separation of the surfaces is a function of surface velocities, load and the viscous properties of the lubricant. These parameters are quantified by hydrodynamic and elastohydrodynamic theory.
Conventionally, oils are supplied to the contacting surfaces as a liquid, in quantities generally sufficient for lubrication and cooling. Cooling is usually necessary because of temperature limitations, which are almost always related to the reduction in viscosity for lubrication or the thermal breakdown of the oil itself. To maintain proper cooling along with lubrication, oils are usually recirculated through a lubrication system with pumps, coolers, sumps, delivery lines and filters. One oil is used with selected viscous properties to cover a specific, but limited, range of temperatures.
This invention provides the process and a means of lubrication of surfaces over a broad temperature range with a small quantity of fluid that can be supplied with a simple system. The invention consists of the lubrication of surfaces with condensed vapors (vapor/condensation, or V/C lubrication) or with vapors that react with the surfaces (vapor/deposition, or V/D lubrication). The condensed vapors are sufficient in thickness to allow EHD film generation and even HD film generation. The amount of lubricant necessary to generate EHD lubricant films is very small, being only a "whisper" of condensate. Vapors that react with the surfaces (see U. S. Pat. 3,978,908 September 1976, E. E. Klaus) are used here as boundary films to supplement V/C lubrication. The preferred mechanism of lubrication is EHD, where the surfaces are completely separated with an oil film under high contact stress, so that the longest possible life of the surfaces can be achieved. The boundary film mechanism is called upon when the EHD mechanism can no longer be maintained.
The invention is most practical with fluids that can be vaporized without leaving breakdown deposits or having the potential for fire. Presently, the most useful fluids are the poly perfluoropolyalky ethers (PFPE) which do not have a flash or fire point and can be vaporized and condensed without solid breakdown products. Another advantage of PFPE's is their availability over a large range of molecular weights or viscosities. This allows a new form of lubrication called variable property (VP) lubrication. VP lubrication provides a condensate film on the surfaces, which increases in molecular weight, or viscosity, with temperature so that HD or EHD lubrication is sustained to an extended temperature level. Chemical additives, which are normally insoluble in PFPE's, can be supplied as vapors to provide boundary lubrication without the limitation of PFPE solubility.
2. Description of Prior Art
Most mechanical components are lubricated with systems which:
provide pressurized jets of oil; PA1 drip feed oil; PA1 provide oil from oil wicks; or PA1 provide oil from an oil sump into which the components dip as they rotate. These systems deliver the oil as a liquid which requires pumps and delivery hardware, except in the case of air-oil mist where misting hardware is required. In each application the selection of a single oil is made to best serve the intended application. Many applications of lubricated components, such as rolling element bearings or gears, require operation over a large range of temperatures and speeds, which go beyond the effective lubrication temperature limits of the oil. To resist this, the oil is used as a coolant to resist thermal breakdown and maintain sufficient viscous properties for lubrication. The lubrication system then becomes complicated with pumping and cooling hardware and large quantities of oil for cooling. High temperature operation within the thermal limits of the oil can be achieved by using high viscosity oils. This, however, usually compromises low temperature pumpability of the oil in the lubrication system. Generally, lubricant selection is based on a compromise between low and high temperature properties. The invention provides a lubrication means not limited by the viscous properties of one oil. The invention allows operation over a broad temperature range with a small quantity of lubricant and little or no cooling.